First you must reach level 25 with each of the four original characters, then Load your level 25 character and create a new one through character management. Hold Turbo and Barbarian becomes Minotaur, and other classes transform into other monsters.
First off, use an auto fire controller and go to the 1st level in the mountain world. Now go to the 1st set of stairs and fix the controller to auto fire at the green trolls coming at you. Leave on overnight for huge levels, sometimes up to 10,000!
